U.S. Pat. No. 3,567,434 is one of the earlier attempts to use high levels of nitrogen in a high chromium duplex stainless steel. The steels have manganese contents below 3%.
Japanese Publication No. 60-181257 discloses a high manganese content (3% to 10%) in a duplex stainless steel having high silicon.
U.S. Pat. No. 4,101,347 includes 0.08% to 0.25% nitrogen in a high chromium duplex stainless steel to stabilize the austenite and improve corrosion resistance. Manganese must be restricted to levels below 2% to avoid sigma phase.
U.S. Pat. No. 4,657,606 adds 5% to 7% manganese to a high chromium duplex stainless steel to simplify the cooling after solution heat treatment. The duplex alloy as 1.1% to 3% copper in the steel composition.
U.S. Pat. No. 4,664,725 has up to 4% manganese added to a duplex stainless steel to stabilize the austenite and provide solubility for nitrogen which can be from 0.06% to 0.20%. At least 2% molybdenum is essential in this steel.